The human body can engage in a vast range of motions. Such motion is constrained according to a variety of tissue types. For example, ligaments are fibrous tissues that can connect bone and can limit the direction and distance of motion in various dimensions that bones can move with respect to one another.
In a specific example, the anterior cruciate ligament (hereinafter, “ACL”) is one of the four major ligaments of the human knee. The ACL originates from deep within the notch of the distal femur. Its proximal fibers fan out along the medial wall of the lateral femoral condyle. The ACL has two bundles: the anteromedial and the posterolateral. Each bundle is named in accordance with where the bundles insert into the tibial plateau.
The ACL attaches in front of the intercondyloid eminence of the tibia, being blended with the anterior horn of the medial meniscus. These attachments allow it to resist anterior translation and medial rotation of the tibia, in relation to the femur.
Various ligaments and other bodily tissues (e.g., the ACL) can be injured in such a way that motions involving such tissues become abnormal. Continuing discussion with respect to the ACL, damage to this ligament is a very common injury, especially in the context of sporting events and other physically rigorous activity. Most often, the ACL is sprained or torn due to rapid, unintentional lateral rotational movements caused by impacts from external forces or falling unexpectedly. Less serious sprains can frequently be addressed by way of physical therapy and muscle strengthening exercises. Tears, on the other hand, almost always require arthroscopic surgery to address.
The health of ligaments and other tissues can be assessed (e.g., to determine injury, to assess recovery after treatment) at least in part by determining the presence or absence of rotational instability and/or abnormal motion. For example, injury to the ACL can be diagnosed by evaluating ligamentous laxity in the knee. The lateral pivot shift test is widely used to evaluate rotational instability after ACL injury and treatment. Traditionally, the result of this test is subjectively determined by the examiner and therefore highly variable. There is presently no widely attainable quantitative evaluation system which can be used in clinical practice despite extensive research efforts to measure rotational instability in the knee or other joints in the human body.
One cause for the lack of an earlier objective evaluation system for rotational instability is the complexity of both normal and abnormal movements. For example, the lateral pivot shift test performed on the ACL evaluates the pivot shift movement. The pivot shift is a six degree-of-freedom movement, including tibial internal-external (i-e) rotation, varus-valgus (v-v) rotation, and anterior-posterior (a-p) translation. Subjective assessment can be used to assign a grade denoting the results of the lateral pivot shift test. Grade I can indicate relatively mild injury that demonstrates abnormal movement when the tibia is held in maximal medial rotation. Grade III (or higher, in some variants) indicates a relatively severe injury that manifests as significant, sudden abnormal movement when the tibia is held in neutral or moderate lateral rotation. Various techniques can attempt to detect abnormal motion by employing invasive navigation systems employing pins fixed into bone to demonstrate correlation between a-p translation of the lateral compartment and the results of the clinical pivot shift test grade.
Most reported devices used in experimental trials of measurements for determining rotational instability (e.g., pivot shift measurement), such as navigation systems and/or electromagnetic sensors and accelerometers, are not universally available. Further, even where available, such devices are costly and require specialized support to practice. Further, although various kinematic assessments have been conventionally used for qualitative evaluation of the pivot shift test, there is no clinically available quantification technique for assessing rotational or other instability. Thus, there is a need in the art for systems and methods to visually quantify such diagnostic values (e.g., quantification of a lateral pivot shift test).